Protective coatings can be either thermoplastic or thermoset. Each type has its own set of advantages and disadvantages. This invention serves to combine the characteristics of both thermoplastic and thermoset coatings so that the advantages of each can be obtained. For example, cellulose ester lacquers are used extensively as coatings for a variety of substrates because they offer good solvent compatibility, are easily applied, sanded and buffed, have good gap-filling properties, low toxicity, good substrate wetting, and numerous other advantages. However, solvent resistance and surface hardness of such coatings are usually poor. Thermoset coatings, on the other hand, offer good hardness and solvent resistance, but they can be difficult to apply or handle and rely on thermal or moisture-type crosslinking mechanisms. Many substrates cannot tolerate high cure temperatures required for the thermal process and moisture-activated systems often have long cure times and humidity requirements.
Photo-activated polymerization of cellulose esters containing ethylenically unsaturated pendent groups has been used to combine the advantages of a thermoplastic lacquer and those of a crosslinked thermoset system. A lacquer coating is applied to a substrate in a solvent solution or by any number of coating processes well-known in the art. At this stage, the lacquer is still thermoplastic. The coating can be repaired or even removed with solvent. On exposure to actinic radiation and in the presence of a photoinitiator, the coating is transformed into a crosslinked polymeric network having enhanced hardness and solvent resistance properties.
Prior art, which teaches such technology, may be found in the U.S. Pat. Nos. 3,749,592, 4,147,603, 4,112,181, 4,404,347, 4,490,516, 4,565,857, and 4,839,230. The cellulose esters described in these patents specifically rely on ethylenically unsaturated polymerizable pendent groups to bond the cellulose ester chains together and/or to co-monomers or oligomers/prepolymers to provide some degree of enhanced coating properties relating to hardness, abrasion resistance, or solvent resistance. The main deficiency of this approach is premature polymerization leading to gelation and contributing to a decreased shelf life requiring stabilization of the cellulose ester by incorporation of free radical inhibitors. These inhibitors also interfere with subsequent photo-initiated polymerization and require increased photo-initiator concentration to overcome their inhibiting effect. Also, polymerization of the acrylate and methacrylate pendent groups described in the above art is inhibited by oxygen and often requires the use of an inert atmosphere such as nitrogen or other inert gas.
An additional deficiency of the prior art involves limited curing and the efficiency of the process. Curing of high molecular weight polymer chains containing ethylenic unsaturation occurs by diffusion of the ethylenic group to the growing chain-end. As the molecular weight of the polymer increases, the diffusion rate decreases and not all the reactive sites are used.
Besides the deficiencies in the above art, individual deficiencies are apparent also. For example, as taught by U.S. Pat. Nos. 4,112,181, 3,749,592, 4,147,603, and 4,490,516, cured films of these cross-linkable cellulose esters never reach adequate levels of solvent resistance and hardness indicative of an incompletely cured network. The photopolymerizable cellulose esters described in U.S. Pat. No. 4,565,857 are uneconomical by requiring reaction of the isocyanate with nearly 100% of available hydroxyl groups. This is required to achieve adequate levels of cure.
Other prior art is described in U.S. Pat. Nos. 3,661,744 and 4,078,118. These describe liquid compositions of polyene and polythiol components that polymerize under UV-curing conditions to give solid film. These do not describe the use of silicon-containing cellulose ester-based thiols. The compositions disclosed in these patents are not polymerized in the solid state as dry films but rather as liquid compositions.
An objective of the present invention seeks to provide cellulose esters with pendent thiol groups which can function as chain-transfer/crosslinking agents useful in coating applications based on free radical polymerization reactions. When acting as chain transfer sites, the pendent thiol groups initiate polymerization. Thus, the thiol groups are not subject to diffusion limitations at a high degree of cure and are effective at developing a fully cured network.